1L SA positions are extremely difficult to get from any school, but if you're talking about diversity positions, it's a bit of a different story.

And it varies by school, but median students with decent interviewing skills should not be struggling to find a 2L SA. That said, people regularly strike out at these schools for a variety of reasons: low grades, bad interviews, inability to apply to the right firms, etc. You should never take a biglaw offer for granted, but the requirements for landing a 2L SA position from Cornell-and-up are much less intense than they are at lower-ranked schools.